This invention relates to a fuel injection pump suited for injecting high-pressure fuel.
In fuel injection pumps disclosed in Japanese Laid-Open (Kokai) Patent Application No. 204963/83 and U.S. Pat. No. 3,885,895, a pump body is made of aluminum in order to achieve a lightweight design of the pump, and a barrel harder than the pump body is used so as to enable the injection of high-pressure fuel. The barrel is mounted in a barrel mounting hole formed in the pump body. A flange is formed on one end of the barrel, and is fixedly secured to the pump body.
The barrel has a cylinder hole and a valve mounting hole which are continuous with each other and are coaxial with the barrel. The inner peripheral surfaces of the cylinder hole and the valve mounting hole are interconnected by an annular flat shoulder surface disposed perpendicular to the axis of the barrel.
A plunger is slidably received in the cylinder hole. A pump chamber is formed by the inner peripheral surface of the cylinder hole and one end face of the plunger.
A delivery valve is mounted in the valve mounting hole. The delivery valve comprises an annular valve seat having a valve port, a valve element for opening and closing the valve port, a spring urging the valve element in a valve closing direction, and a valve holder. The valve holder is threaded into the valve mounting hole. The valve seat is interposed or held between the shoulder surface of the barrel and the valve holder.
Fuel in the pump chamber is pressurized or compressed by the plunger, and is fed under pressure to a fuel injection nozzle, mounted on an engine, via the delivery valve.
Generally, a requirement for the barrel is that the inner peripheral surface of the cylinder hole will not be subjected to premature wear due to a high-speed sliding movement of the plunger. To meet this requirement, in the above Japanese Laid-Open Patent Application No. 204963/83, the hardness of the inner peripheral surface of the cylinder hole is not less than HRc60 (not less than HV697), and in U.S. Pat. No. 3,885,895, this hardness is HRc61(HV720).
In the above fuel injection pumps, it is necessary to prevent the high-pressure fuel in the pump chamber from leaking to the exterior through the area of contact between the shoulder surface of the barrel and the valve seat. To achieve a good seal between the barrel shoulder surface and the valve seat, two requirements must be met. A first requirement is that the valve holder should be threaded into the valve mounting hole with a high force so as to hold the valve seat in firm contact with the barrel shoulder surface. Since this threading force does not act on the pump body of a low strength, the first requirement can be met easily. A second requirement is that the valve seat and the barrel shoulder surface should be held in intimate face-to-face contact with each other over the entire areas thereof. This requirement can not easily be met. The reason is that since the shoulder surface is formed in the inside of the barrel, it is difficult to obtain highly-precise flatness of the shoulder surface, as well as highly-precise perpendicularness of the shoulder surface relative to the axis of the barrel, by grinding.
In the fuel injection pump of U.S. Pat. No. 3,885,895, a gasket of a copper alloy is interposed between the shoulder surface of the barrel and the valve seat of the delivery valve. The copper alloy gasket is relatively soft, and therefore even if the flatness and perpendicularness of the shoulder surface are somewhat low in precision, the gasket is elastically deformed, involving a local plastic deformation, so that the opposite sides of the gasket are brought into contact with the barrel shoulder surface and the valve seat over generally the entire areas thereof, thereby achieving good sealing properties. However, this construction is not suitable when it is required to pressurize the fuel to a higher degree. The reason for this will now be described. In order to seal the fuel of greater pressure, it is necessary to hold the gasket in contact with the barrel shoulder surface and the valve seat under greater pressure. In this case, however, the gasket is plastically deformed over the entire area thereof, and therefore fails to increase the contact pressure. In the worst case, the gasket is damaged or ruptured.
In the above Japanese Laid-Open Patent Application No. 204963/83, no gasket is used, and the shoulder surface of the barrel is in direct contact with the valve seat. With particular reference to FIG. 2 of this prior Japanese application, the hardness of the barrel shoulder surface is HRc45 to HRc60 (Vickers hardness: HV446 to HV697), and even if the valve seat is strongly urged against the barrel shoulder surface, no local plastic deformation occurs. Therefore, an error in flatness of the shoulder surface as well as an error in perpendicularness of the shoulder surface relative to the axis of the barrel, can not be absorbed, thus failing to achieve good sealing properties.
In the above U.S. patent and the above Japanese application (FIG. 2), besides the hardness of the inner peripheral surface of the cylinder hole in the barrel, the hardness of the threaded portion of the valve mounting hole is improved. The threaded portion is soft to have such toughness as to withstand a reaction force applied from the valve seat. More specifically, the barrel is heat-treated in such a manner as to create a hardness gradient in the direction of the axis thereof, so that that region of the barrel including the cylinder hole is the highest in hardness whereas that region of the barrel including the threaded portion is the lowest in hardness. The hardness of the threaded portion of the above U.S. patent is below HRc36 (below HV354), and the hardness of the threaded portion of the above Japanese application is HRc35 (HV345). However, the hardness of the barrel shoulder surfaces of these prior art is too high, and is not suitable for enhancing the sealing properties. Moreover, the barrel shoulder surface is continuous with the inner peripheral surface of the cylinder hole, and therefore the intended hardness of the barrel shoulder surface which is an important feature of the present invention can not be obtained by the heat treatment causing a gentle hardness gradient in the direction of the axis of the barrel as in the above prior art.